The number of African Americans residing in the District plummeted by more than 11 percent during the past decade, with blacks on the verge of losing their majority status in the city for the first time in half a century. According to census statistics released Thursday, barely 50 percent of the District’s population was African American in 2010 — a remarkable shift in a place once nicknamed “Chocolate City.” The black population dropped by more than 39,000 over the decade, down to 301,000 of the city’s 601,700 residents.